Paul Smith on the Cambrian Explosion
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As he explains in the podcast, Paul Smith believes that we need to bring diverse research disciplines to bear on the question of life’s explosive proliferation between 541 and 530 million years ago during the Cambrian period. These disciplines include paleobiology, sedimentology, and geochemistry. He suggests that only when sufficiently evolved genetics and conducive environmental conditions coincided could complex feedback loops begin to operate and drive explosive growth in the abundance and diversity of life.
Smith is Director of the Oxford University Museum of Natural History and Professor of Natural History at the University of Oxford. The photo was taken after a day of excavating at Sirius Passet in northern Greenland.
Courtesy of Jan Audun Rasmussen
Podcast Illustrations
Images courtesy of Paul Smith unless otherwise indicated.
Late Neoproterozoic to Early Phanerozoic Time Scale
Geological timescale of animal origins. In the two major Snowball Earth glaciations in the Cryogenian period, life survived in limited areas of unfrozen ocean and melt-water ponds on the surface of the ice that covered nearly all the planet. As discussed by Paul Hoffman in his episode of Geology Bites, all modern life is thought to be descended from these organisms. In this episode, Paul Smith talks about the emergence of much more complex life about 100 million years later in the Ediacaran and Cambrian.
Diversification
The major diversification of marine taxa between 635 and 443 million years ago. The red box indicates the time interval discussed in the podcast. EB: Ediacaran biota.
Tree of Life exhibit at the Oxford University Museum of Natural History. Today’s living species are shown at the outermost reaches of the tree. Moving back in time towards the center of the tree, the branching points show the last common ancestor between species. The diversification of the Cambrian explosion appears about halfway to the center (not to scale) just after the divergence of the chordates, molluscs, and arthropods. The tree was constructed using a combination of genetic information, which, as Smith explains in the podcast, provides the scaffolding or framework of the tree, and morphological data from fossils that informs the morphological evolution of groups in their early history. To learn about the evolution of body plans of animal groups that populate the tree of life, we must rely on a combination of genetics and paleobiology.
Interconnected Causes
In the podcast, Smith suggests that no single process is responsible for the Cambrian explosion. Tectonic and geological, developmental, and ecological processes have been hypothesized as isolated, singular causes. Instead, many of these processes are parts of connected feedback loops that together generated the Cambrian explosion. Each box corresponds broadly to a stand-alone hypothesis or suite of related hypotheses (red, geological; blue, geochemical; green, biological). The red arrows connect the explosion in animal diversity to five proximal causes.
The figure represents the interval of time at the beginning of the Cambrian, 541 million to 521 million years ago.
Smith, M.P., et al. (2013), Science 341, 1355
Paleogeography
The configuration of the continents following the breakup of the Rodinia supercontinent about 750 million years ago. The locations of the sites where exceptionally well-preserved beds (Lagerstätten) of Cambrian fossils have been found are marked. Today, the Burgess Shale is in the Canadian Rockies, Sirius Passet is in the far north of Greenland, and Chengjiang is in southern China.
cpgeosystems.com
Burgess Shale
The most famous of the Burgess Shale Formation members at the Walcott Quarry. It dates from 508 million years ago, about 10 million years younger than the Sirius Passet locality. Outcropping at an elevation of 7,500 ft. in the Canadian Rockies, it was discovered by paleontologist Charles Walcott in 1909.
Photo: Mark A. Wilson
Artist’s impression of the deep-water shelf environment where the Burgess Shale biota lived.
Courtesy of D.W. Miller
The Sirius Passet Fossil Bed
Satellite view of North America and Greenland showing the location of Sirius Passet. This location is one of the few known Cambrian Lagerstätten. Their fossils contain preserved soft-tissue structures that provide an important window to the evolution of animals at the time of the Cambrian explosion.
NASA
Geology of the Sirius Passet locality. The fossils lie in the laminated mudstones and muddy siltstones of the Cambrian formation at left. These are bounded at right by a fault where they are in contact with older white carbonate rocks of the uppermost Ediacaran. The fossil quarry site is marked by the red rectangle. At the left end of the rectangle, members of Smith’s team can just be seen.
Left: searching for fossils in the mudstone scree.
Above: preparing the samples for the flight south.
Some of the fauna from the Sirius Passet locality (scale bar = 1 cm). The fauna are dominated by arthropods, some of which lived on the microbial mat surfaces (e.g., a and b left) together with sponges (j) and others in the water column above (e.g., f left, a right). Rarer elements of the fauna include loriciferans (h), palaeoscolecid worms (i), vetulicolians (b right) and polychaete worms (c right).
Harper, D.A.T., et al. (2019), Journal of the Geological Society 176, 1023
As described in the podcast, all the sites of exceptional preservation in the Cambrian appear to have been located at the deep end of a continental shelf where light and nutrients were limited, but where the substrate was stable and free from disturbance by the action of waves and tides. The depositional location of the Sirius Passet Lagerstätte is indicated by a red star.
Harper, D.A.T., et al. (2019), Journal of the Geological Society 176, 1023
Vetulicolians
Left: Vetulicolians from Sirius Passet. The scale bar is 5 cm.
Vinther, J. et al. (2011), Paleontology, 54, 711
Above: artist’s impression of a vetulicolian colony. In the podcast, Smith describes these creatures as having a tail at one end and an open crisp (potato chip) packet at the other.
Courtesy of Katherine Child
Further Reading
Smith, M.P., et al. (2013), Science 341, 1355
Hammerlund, E.U., et al. (2018), Geobiology, 17, 12
Harper, D.A.T., et al. (2019), Journal of the Geological Society 176, 1023
Smith, M.P. (2020) The Cambrian Explosion and the evolutionary origin of animals: Oxford University Museum of Natural History talk: https://www.youtube.com/watch?v=blN-F8n2IAc